1. Field of the Invention
The present invention relates to a method and apparatus for treating wastes by gasification, and more particularly to a method and apparatus for treating wastes by two-stage gasification to recover metals or ash content in the wastes in such a state that they can be recycled and gases mainly composed of carbon monoxide (CO) and hydrogen gas (H2) for use as synthesis gas for hydrogen gas or ammonia (NH3).
2. Description of the Prior Art
Ammonia is a mass-produced basic material for producing nitric acid, fertilizers including ammonium nitrate, ammonium sulfate and urea, acrylonitrile, caprolactam or the like. Ammonia is catalytically synthesized from nitrogen gas (N2) and hydrogen gas (H2) under a high pressure. Hydrogen gas has been produced by either steam reforming of natural gas or naphtha, or partial combustion, i.e., gasification of hydrocarbon such as crude oil, heavy oil, bottom oil, coal, pitch or petroleum coke.
Since most of materials for producing hydrogen gas are dependent on importation from abroad, ammonia-derived chemical products have lost their competitiveness on the world market after recent oil crises. Therefore, there has been a strong desire for obtaining materials which are inexpensive and available domestically.
It has heretofore been customary to treat organic wastes such as municipal wastes, plastic wastes including fiber-reinforced plastics (FRP), biomass wastes, and automobile wastes by incineration to reduce their volume for reclaiming, or to discard them in an untreated state to landfill sites.
Therefore, only a small quantity of resources has been recovered from the organic wastes for the purpose of recycling, irrespective of direct or indirect utilization.
On the other hand, the incineration of organic wastes has been disadvantageous for the following reasons:
A stoker furnace or a fluidized-bed furnace has heretofore been used for the incineration of organic wastes. However, this incineration has been problematic with respect to environmental conservation, or recycling of resources or energy. To be more specific, large quantities of exhaust gas are discharged because of a high air ratio, and toxic dioxins are contained in the exhaust gas. Further, metals which are discharged from the furnace are not suitable for recycling because they are severely oxidized, and landfill sites become more scarce year by year. Recently, the number of waste treatment facilities which incorporate ash-melting equipment is increasing, however, a problem is encountered in the increase of construction costs and/or operating costs of the waste treatment facilities. Further, recently, there has been developing a tendency to utilize energy of the wastes more efficiently.
Dumping the wastes in an untreated state on reclaimed land has become more difficult because of scarcity of landfill sites, and has not been allowable from the viewpoint of environmental conservation. Therefore, the problem of disposing of harmful wastes such as shredder dust from scrapped cars by landfill is getting worse and worse.
Further, in the case where a large quantity of steam is used with oxygen gas (O2) as a gasifying agent in the fluidized-bed reactor, the operating cost increases. Even if air which is easily available is used as the gasifying agent, there is a limit to the amount of air because a limitation on the amount of nitrogen that can be used for synthesis of ammonia.
It is therefore an object of the present invention to provide a method and apparatus for treating wastes by two-stage gasification which can recover resources in the wastes for the purpose of recycling, produce synthesis gas having components for use as synthesis gas for ammonia by partial combustion, solve various problems caused by incineration or landfilling of organic wastes, and obtain a low cost synthesis gas for hydrogen gas or ammonia.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for treating wastes by gasification, comprising: gasifying wastes in a fluidized-bed reactor at a relatively low temperature; introducing gaseous material and char produced in the fluidized-bed reactor into a high-temperature combustor; producing synthesis gas in the high-temperature combustor at a relatively high temperature; converting the synthesis gas by a CO-shift reaction after scrubbing for removal of acid components; producing hydrogen gas by a gas separation process; and supplying residual gas to the fluidized-bed in the fluidized-bed reactor.
The gas separation process may be carried out by one of pressure swing adsorption or a hydrogen gas separation membrane.
According to another aspect of the present invention, there is provided a method for treating wastes by gasification, comprising: gasifying wastes in a fluidized-bed reactor at a relatively low temperature; introducing gaseous material and char produced in the fluidized-bed reactor into a high-temperature combustor; producing synthesis gas in the high-temperature combustor at a relatively high temperature; converting the synthesis gas by a CO-shift reaction; removing acid components in the gas from the CO-shift reaction to obtain hydrogen gas; and supplying a part of the removed acid components to the fluidized-bed reactor. The above-mentioned CO-shift reaction can be carried out after removal of the acid components in the synthesis gas.
According to still another aspect of the present invention, there is provided an apparatus for treating wastes by gasification, comprising: a fluidized-bed reactor for gasifying wastes at a relatively low temperature to produce gaseous material and char; a high-temperature combustor for producing synthesis gas at a relatively high temperature, a cooling chamber containing water for cooling the synthesis gas; a water scrubber for removing acid components from gas supplied from the cooling chamber; a CO-shift converter for carrying a CO-shift reaction to convert CO, H2O in the gas from the water scrubber into CO2, H2; a gas separator for separating gas into hydrogen gas and residual gas; and a pipeline for supplying the residual gas to the fluidized-bed reactor.
According to still another aspect of the present invention, there is provided an apparatus for treating wastes by gasification, comprising: a fluidized-bed reactor for gasifying wastes at a relatively low temperature to produce gaseous material and char; a high-temperature combustor for introducing synthesis gas at a relatively high temperature, a cooling chamber containing water for cooling said synthesis gas; a CO-shift converter for carrying out a CO-shift reaction to convert CO and H2O in the gas from said cooling chamber into CO2 and H2; an acid gas remover for removing acid components in the gas after the CO-shift reaction; and a pipeline for supplying a part of said acid components to the fluidized-bed reactor.
The acid gas remover may be provided between the cooling chamber and the CO-shift converter.
The two-stage gasification in the present invention may be carried out under atmospheric pressure, but economically may be carried out under a pressure ranging 5 to 90 atm, preferably 10 to 40 atm. As a gasifying agent, air and/or oxygen gas obtained by air separation may be used. Alternatively, steam or carbon dioxide (CO2) may be added to them.
The fluidized-bed temperature of the fluidized-bed reactor is preferably in the range of 450 to 950xc2x0 C. The wastes preferably have an average lower calorific value of 3500 kcal/kg or more. If the average lower calorific value of the wastes is 3500 Kcal/kg or less, a supplementary fuel may be added to the wastes to allow the average lower calorific value to be 3500 kcal/kg or more. As a supplementary fuel, fossil fuel, which is generally used, such as coal or petroleum coke, may be used.
As a fluidized-bed reactor used in the present invention, a revolving flow-type fluidized-bed reactor is preferably used. In the revolving flow-type fluidized-bed reactor, a revolving flow of the fluidized medium is formed in the fluidized-bed by controlling linear velocity of fluidizing gas. The revolving flow-type fluidized-bed is superior in the functions of dispersion and crushing of char to a bubbling-type fluidized-bed in which linear velocity of the fluidizing gas is uniform. Further, the revolving flow-type fluidized-bed reactor is structurally simpler and smaller-sized, compared to an externally circulating fluidized-bed reactor. The revolving flow-type fluidized-bed reactor preferably has a vertical cylindrical shape because it is operated under pressurized conditions.
In the high-temperature combustor, gaseous material containing ash and char produced in the fluidized-bed reactor are gasified at a temperature higher than an ash melting point. The temperature in the high-temperature combustor is 1200xc2x0 C. or higher.
In the present invention, the total amount of oxygen gas supplied to the fluidized-bed reactor and the high-temperature combustor may be in the range of 0.1 to 0.6 of the theoretical amount of oxygen required for combustion. The amount of oxygen supplied to the fluidized-bed reactor may be in the range of 0.1 to 0.3 of the theoretical amount of oxygen required for combustion.
The fluidized-bed reactor has a reducing atmosphere, and thus metals in the wastes can be recovered in a non-corroded condition from the bottom of the fluidized-bed reactor. Further, the temperature in the high-temperature combustor is set at 1200xc2x0 C. or higher so that the temperature in the high-temperature combustor is 50 to 100xc2x0 C. higher than the ash melting point. Thus, the content is discharged as molten slag from the bottom of the combustor.
In the present invention, a gas separation unit for separating air into nitrogen gas and oxygen gas is provided. In the case of producing ammonia, means for supplying the separated nitrogen gas to an ammonia synthesis reactor, and means for supplying the separated oxygen gas to the fluidized-bed reactor and/or the high-temperature combustor may be provided.
Wastes which are used in the present invention may be municipal wastes, plastic wastes including fiber-reinforced plastics (FRP), biomass wastes, automobile wastes, low-grade coal, waste oil, and alternative fuels such as RDF (refuse-derived fuel) and SWM (solid-water mixture) made from the above wastes.
The alternative fuels include refuse-derived fuel which is produced by pulverizing and classifying municipal wastes, adding quicklime, and compacting for pelletization, and solid-water mixture which is produced by crushing municipal wastes, adding water and mixing, and converting to an oily fuel by a hydrothermal reaction. The biomass wastes include wastes from waterworks or sewage plants (misplaced materials, screen residues, sewage sludges, or the like), agricultural wastes (rice husk, rice straw, surplus products, or the like), forestry wastes (sawdust, bark, lumber from thinning, or the like), industrial wastes (pulp-chip dust, or the like), and construction wastes. The low-grade coal includes peat which has a low degree of coalification, or refuse from coal dressing.
The present invention is also applicable to organic materials including oil shale, garbage, carcasses of beasts, waste clothing, waste paper, and any other material.
The above and other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.